This invention relates to methods and arrangements for NO.sub.X exhaust emission control for internal combustion engines.
As used herein in connection with gas storage catalysts, the term "absorb" includes the chemical process for storing gases such as, for example, by conversion of barium oxide to barium nitrate for storage of nitrogen oxide.
British Patent No. 2,290,488 describes a spark-ignited internal combustion engine having an emission control arrangement with a main catalytic converter and an upstream catalytic converter. The main catalytic converter is a conventional three-way catalytic converter with a washcoat containing cerium which serves as an oxygen accumulator. The upstream catalytic converter has an upstream zone with a washcoat that contains The upstream catalytic converter has an upstream zone with a washcoat that contains palladium but no cerium. The upstream zone is followed by a conventional three-way catalyst with a platinum and rhodium coating. The internal combustion engine is operated with a stoichiometric mixture, i.e. at .lambda.=1. This catalytic coating arrangement makes the catalytic converter more stable as compared to arrangements subjected to rich and lean exhaust gas conditions rapidly fluctuating about .lambda.=1. This exhaust emission control arrangement, however, is not useful in lean-bum internal combustion engines, such as lean mix Otto engines and Diesel engines, since such exhaust emission control devices are only stable with respect to aging at a theoretical air/fuel ratio .lambda.=1.
Moreover, there are conventional NO.sub.X storage catalytic converters for lean-burn Otto engines which during lean operation initially store the nitrogen oxides up to a load limit which is dependent on the catalytic converter design. This is followed by a brief stoichiometric or slightly rich operation for regeneration of the NO.sub.X storage catalytic converter, after which lean burn operation is restored. The degree of NO.sub.X retention in these NO.sub.X storage catalytic converters is very high, i.e. the overall NO.sub.X reduction with accumulator discharge and NO.sub.X conversion at .lambda..ltoreq.1 is &gt;90% for lean-burn Otto engines in new condition. In principle, such NO.sub.X storage catalytic converters can also be used with Diesel engines, wherein a certain increase in the catalyst dimensioning is advantageous to allow for SO.sub.X storage. In contrast to Otto engines, however, Diesel engines always operate with excess air so that .lambda.&gt;1 during all operating states. Loading of the NO.sub.X storage catalytic converter is possible without any difficulty, but regeneration by enriching the exhaust gases, for example by injecting fuel into the exhaust line, would lead on the one hand to an intolerable increase in fuel consumption and on the other hand to a high oxidation heat due to the high oxygen content of the diesel exhaust since the injected fuel would be oxidized prior to the conversion of the stored NO.sub.X. This results in the risk of destruction of the catalytic converter.
U.S. Pat. No. 5,406,790 discloses an exhaust emission control device for Diesel engines in which the NO.sub.X accumulator is shut off from the exhaust gas stream for regeneration. This takes place whenever the accumulator has reached its capacity. In order to avoid NO.sub.X emissions, the exhaust gas stream is directed to a second NO.sub.X accumulator during regeneration. Alternatively, the exhaust gas stream is throttled and a complicated regeneration algorithm is initiated. Functional reliability is problematic in this context. Moreover, duplication of the NO.sub.X accumulator involves considerable expense and good exhaust emission control is achieved only under certain conditions despite the cost.
In U.S. Pat. No. 4,755,499 reversible storage of nitrogen oxides and sulfur oxides, for instance from motor vehicle exhaust emissions, is described, wherein the absorbent is regenerated by heating in a reducing atmosphere. At the same time, a reduction of the nitrogen oxides occurs.
A storage catalyst of that type for use in motor vehicles is described in more detail in U.S. Pat. No. 5,402,641, in which high temperatures above 500.degree. C. are necessary to regenerate the absorber. Consequently, use of the storage catalyst is possible only for motor vehicles having a high exhaust-gas temperature, in particular for motor vehicles with an Otto engine.
In this case, however, the possibility of use is limited since, under certain operating conditions of internal combustion engines, such as occur for example in city traffic, the acceleration phases cause a large emission of nitrogen oxide, but no long-lasting high temperature condition such as is required to regenerate the absorber, especially with respect to oxides of sulfur, is attained.
U.S. Pat. No. 5,473,887 discloses an Otto engine with an NO.sub.X storage catalyst as well as one three-way catalytic converter upstream and another three-way catalytic converter downstream from the storage catalyst. Due to the proximity of the upstream three-way converter to the engine, that converter heats up very quickly after a cold start so that it starts its catalytic activity promptly. After starting its catalytic activity, the upstream three-way catalytic converter converts the increasing quantities of HC and CO produced during the warming-up phase of the engine while reducing NO.sub.X at the same time. As a result, the NO.sub.X in the exhaust gas is reduced during a warming-up phase of the engine, even though the NO.sub.X accumulator has not yet achieved the temperature necessary for storage of NO.sub.X. This arrangement and method make sense only for Otto engines since Diesel engines do not emit enough CO and HC to sufficiently reduce the NO.sub.X portion of the exhaust gases even during the warming-up phase. Except for the improved exhaust emission control during the warming-up phase, an Otto engine with an upstream catalytic converter does not demonstrate any better NO.sub.X reduction.
U.S. Pat. No. 5,473,887 also discloses two methods for NO.sub.X reduction in Diesel internal combustion engines, one based on throttling of the air supply to the engine and the other based on control of the fuel injection. Diesel internal combustion engines that are equipped with NO.sub.X accumulators as described above, however, show a definite reduction in NO.sub.X storage in the NO.sub.X accumulator at higher exhaust gas temperatures.
Common to all these methods and arrangements is that running a rich mixture in an internal combustion engine for regeneration of the NO.sub.X is often problematic, especially in direct-injection internal combustion engines and/or Diesel engines. Thus, in this context, the temperature of the exhaust gases can be too high on the one hand, or, on the other hand, the resultant reduction in output power can be too large.
Moreover, depending on the size of the NO.sub.X accumulator, flow-through can occur, that is some NO.sub.X flowing into the NO.sub.X accumulator is not absorbed even though sufficient storage capacity is available or some of the hydrocarbons that flow into the NO.sub.X accumulator for regeneration thereof exit from the NO.sub.X accumulator again.